According to the documentation, the vectorY property of 8Direction should be settable, however, it does not seem to have an effect when done through javascript.
console.log("VERTICAL");
var lineUP = this.runtime.objects.Laser.createInstance("PlayBoard", thisPiece.x, thisPiece.y);
lineUP.behaviors["8Direction"].vectorY = 100;
console.log(lineUP.behaviors["8Direction"].vectorY);
console.log("X:" + lineUP.behaviors["8Direction"].vectorX + " Y:" + lineUP.behaviors["8Direction"].vectorY);